1. Field of the Invention
The present invention relates to an optical information recording medium suited for optical writing recording by use of a laser beam or the like, and, more particularly, to an improved optical recording medium that can be used in optical discs, optical cards, etc.
2. Related Background Art
In general, optical recording mediums as exemplified by optical discs and optical cards can record information in a high density by forming optically detectable pits of minute size, for example, of about 1 .mu.m on a thin recording layer provided on a substrate having grooves of spiral, circular or linear form.
The recording layer, i.e., a laser-beam-sensitive layer can absorb laser beam energy to form optically detectable pits thereon. For example, in a certain heat mode recording system, the laser-beam-sensitive layer absorbs a heat energy to form minute concaves (i.e., pits) on that energy-absorbed parts by evaporation or fusion. In another heat mode system, the absorption of the energy of the irradiated laser beam can form pits having an optically detectable density difference on that parts.
The information recorded on the optical discs or optical cards can be detected by reading optical changes between the part on which the pits are formed and the part on which the pits are not formed. For example, in the instance of the optical discs or optical cards, a laser beam is scanned along a track, and the energy reflected by a disc or card is monitored by a photodetector. At the part on which the pits are formed, the reflection of the laser beam is lowered and the output from the photodetector becomes smaller. On the other hand, at the part on which the pits are not formed, the laser beam is sufficiently reflected and the output form the photodetector becomes larger.
Hitherto proposed as the recording layers used in such optical recording mediums are those in which inorganic materials are chiefly used, for example, metallic thin films such as aluminum- or gold-deposited films, bismuth thin films, tellurium oxide thin films, chalcogenite type amorphous glass films or the like. These thin films, however, have been disadvantageous such that they involve poor storage stability, low resolution power, low recording density, high production cost, etc.
Recently, also proposed is to use in the recording layer an organic coloring matter thin film whose physical properties can be changed by light of a relatively long wavelength. This organic coloring matter thin film can eliminate the above disadvantages, but, in general, organic coloring matters having absorption characteristics on the side of the long wavelength has the problem such that they have a low stability to heat and light. Taking account of these points, the organic coloring matters used in recording materials are required to have the following properties. Namely;
1. They have no toxicity;
2. They have absorption in the vicinity of 800 nm, and have a large absorptivity coefficient;
3. They have a good solubility to organic solvents;
4. They have a large reflectance in the vicinity 20 of 800 nm in a thin film state;
5. They can be crystallized with difficulty in a thin film state;
6. They have stability to ultraviolet light and visible light;
7. They have thermal stability;
8. They have stability to moisture;
9. They can be readily synthesized; etc.
Various types of organic coloring matters have recently been proposed as the coloring matters that can satisfy these performances. However, these organic dyes, although they are compounds that can be utilized in optical recording mediums, have the problem that the CN value may be lowered by repeated irradiation of reproducing light, which is a disadvantage generally inherent in the organic recording materials. This is presumably because heat is accumulated in the recording layer as heat energy when even a laser beam of low energy like the reproducing light is repeatedly irradiated, and the recording layer is fused or decomposed by that heat.
On the other hand, it has been reported in various ways to provide a subbing layer in the optical recording medium for the purposes of improving the adhesion between a substrate and a recording layer, improving solvent-resistant effect and reflectance to the substrate, and improving storage stability of the recording layer. Known materials for the subbing layer include polymeric materials such as polyamide type resins, vinyl type resins, natural polymers and silicone resins, or silane coupling agents, inorganic compounds such as MgF.sub.2, SiO, TiO.sub.2, ZnO, TiN and SiN, and metals such as Zn, Cu, S, Ni, Cr and Se.
The subbing layer made of these materials, however, aims at improving the adhesion and solvent resistance mentioned above, and there have been known not so many subbing layers that are effectual for obtaining a high CN value (or SN value), except that, for example, Japanese Laid Open Patent Application No. 11292/1986 discloses, as a subbing layer that can improve the CN value, a subbing layer comprising a coating of a colloid particle dispersion with a silicon type condensate. The optimum film thickness range of this subbing layer, however, is very narrow as little as from 80 .ANG. to 120 .ANG., and the CN value may greatly vary depending on the irregularity in the film thickness, resulting in difficulty in controlling the film thickness when preparing the recording medium.
On the other hand, the recording layer is known to have better pit shapes when the recording is performed, if it is in a flat state as nearly as possible. When the recording is performed on a recording layer on which the shape of truck grooves is reflected, the rims of the pits formed are liable to lose their shapes to worsen the pit shapes, therefore leading to worsened CN values.
Accordingly, the subbing layer may preferably be formed on the substrate with a little larger thickness in the manner that the recording layer to be laminated thereon may become close to a flat state. However, in the case of the above-mentioned subbing layer, an increase in the film thickness may result in the worsening of CN values. This is presumably because the subbing layer comprising the silicon type condensate has so a high thermal conductivity that it may follow, with increase in the film thickness, that the heat energy generated when the power of a recording laser beam is converted from light to heat in the recording layer is not effectively used for the formation of the pits. Moreover, if the film thickness is made to be from 500 .ANG. to 1,000 .ANG. or more, it follows that the track grooves provided on the substrate, which are usually formed thereon with a depth of 500 .ANG. to 1,000 .ANG., are buried and no tracking signal is obtained, resulting in reproduction incapability.